Like mini-skirts and hot tubs, the trend toward fewer heart disease deaths started on the coasts and worked its way to America's heartland.

"There turns out to be a tremendous amount of geographic variation in when the drop in heart disease began," said Dr. Steven Wing, an epidemiologist at the University of North Carolina at Chapel Hill, in announcing his study. The patterns Wing uncovered could help settle disputes about what has been responsible for the improved condition of American hearts.

Since the early 1960s, heart disease rates have declined 30 percent overall, but 50 percent in some parts of the country. Competing theories credit more exercise, more effective surgery and medical treatments, and improved diets for the decline in deaths.

Wing's study, to be published soon in the American Journal of Public Health, looked at death rates in 200 cities and 300 rural areas across the country from 1968 through 1978.

While heart disease rates in cities of the Northeast and West declined steadily over the 11 years, rates in the rural South and other areas climbed through the early 1970s before turning around.

Comparison with diet patterns, types of jobs and other life style factors could help determine which plays the largest role in heart disease, Wing said. New Bone Joint Surface Could Lengthen Devices' Life

A new way to attach replacement joints to bones may one day extend their life and make them available to younger patients.

Some 200,000 joints -- mostly hips and knees -- are replaced in the United States each year. Usually, they are secured with cement, but the cement can give way after 10 to 12 years, and repairs are difficult, experts say.

The new surface designed by Dr. Steven A. Goldstein and other researchers at the University of Michigan eliminates the need for cement and, they hope, will better distribute weight along the bone.

Cones on the surface of the joint are tapped into the bone. Because the joint is made of porous materials, the bone should be able to form a bond with it. In theory, the amount of pressure on the bone varies along the cones, allowing the bone to grow to the most comfortable part of the cone.

Other implants, without the conical design, are being tested in various research hospitals. Goldstein hopes to begin animal trials on his design soon. Skin-Stretching Tried As Baldness Treatment

A new skin-stretching technique has successfully shrunk bald areas in 27 people, a California doctor says.

Dr. Sheldon Kabaker, a plastic surgeon at the University of California at San Francisco, adapted the technique, called tissue expansion, for use on the scalp. Tissue expansion occurs naturally in pregnancy when the skin stretches to accommodate the growing fetus, and also has been used to repair scars.

"Similar techniques have been used for centuries by some African cultures for enlarging certain facial features," he writes in the Archives of Otolaryngology -- Head and Neck Surgery.

Small balloons are implanted in the scalp where hair is still growing. Over six to eight weeks, the balloons are slowly inflated with fluid, causing the skin to stretch. Then the baloons are removed, the bald skin is cut away and the flap of stretched skin replaces it.

"The hair follicles become separated from one another during the process of expansion, but the number of follicles remains the same," Kabaker reports.

The study involved 17 people, and 10 more have undergone the treatment since the findings were submitted, he said.

Many of the patients objected to being seen in public with the bulges created on their head during the six to eight weeks of tissue expansion. "This appears to be the most negative aspect of the procedure," Kabaker reports. Most of the patients wore hats in public during that time. Natural Disasters Linger In Minds of Children

Ten months after a natural disaster affecting their lives, fifth-grade girls showed signs of emotional distress but boys the same age did not, psychiatrists have found.

This may indicate that boys of that age are practicing denial as a way of coping with the disaster.

Ten months after a blizzard and flood paralyzed parts of Revere, Mass., and forced thousands to evacuate their homes during a week-long official state of emergency, psychiatrists asked children in one church to write essays on "What This Coming Winter Will Be Like." Another group of children from a neighborhood not severely affected wrote essays for comparison.

A typical essay from the nonflooded group began: "I hope this winter will be the best," while one from the flooded group said: "There will be another blizzard . . . For people down on the beach it will be bad . . . people will get hurt."

Essays by girls from the flooded area showed significantly more depression and preoccupation with dying than those by girls from the non-flooded area, the psychiatrists, headed by Dr. Jack D. Burke Jr of the National Institute of Mental Health, concluded.

Writing in the Journal of the American Academy of Child Psychiatry, Burke and his colleagues said girls of that age may have written more about their fear of disaster because they have better language skills than boys. Or the boys, for some reason, may be trying "to suppress anticipatory anxiety as winter returns." Illness Reported From Injection of Marijuana Broth

There have been 25 reported instances of people injecting marijuana broth and becoming ill since 1968, and doctors continue to report new cases.

Drs. Daniel Brandenburg and Richard Wernick of Providence Medical Center in Portland, Ore., describe the symptoms of "intravenous marijuana syndrome" in the current Western Journal of Medicine. Symptoms include coughing, diarrhea, vomiting, leg pain, fever and lowered blood pressure.

The doctors treated a 26-year-old man for those symptoms and found descriptions of 24 other cases in the medical literature. All the patients recovered within a few days.

It is still unclear exactly what causes the symptoms, the doctors said. In one study, in which cannabis, the active ingredient of marijuana, was given intravenously, the psychological symptoms associated with smoking the drug occurred, but not the illness.

The doctors conclude that dosages may be higher when the drug is injected outside of a clinical setting, or that contaminants, such as cotton frequently used to strain the broth, may be partly responsible. On the Pulse

Women outnumber men three to one in health jobs, but are clustered at the low end of the pay scale, a University of Michigan study concludes. Women hold 87 percent of the health jobs that pay less than $20,000 a year . . . Acyclovir, the new herpes drug, worked well on four patients who had a relatively rare rash on their hands and legs related to the virus, the American Academy of Dermatology reports. Some of the patients were able to use the drugs for longer than the recommended six months without side effects, the study found . . .